@Roger the laptop is pretty user friendly and easy to open, just unscrew the bottom plat screws (pay attention which one goes where, the L15 has three different sizes) and take off the bottom plate - aka D-shell. RAM and the two SSD slots (M.2 and 2.5") are easy to identify.
When handling electronic components please make sure that you are doing this in an as ESD safe environment as possible, do not directly touch any connectors (hold parts on their edges / corners), ground yourself before starting the work (prepare a clean table and chair), sit down and then firmly touch a metal part connected to AC earth for some seconds.
For opening the case and handling the parts (especially storage) you need a Philips type cross headed screwdriver PH #00.
While closing the laptop pay attention that all side of the D-shell go flat onto the C-shell again - it happens pretty easily that it does not snap down fully when re-attaching. Look at the laptop before opening and remember how it looks, that’s what it should look like again after the operation
Concerning parts, RAM is DDR4 SODIMM 2133 or 2400, up to 32GB. The 2.5" slot can carry pretty much any 2.5" SSD or harddisk (SATA) with a height of up to 9.5mm. The M.2 slot is a M.2 2280 type and is compatible with M.2 SATA and M.2 NVME SSDs. The NVME offers best performance.
Be careful - and have fun